A cluster-based multiple deep neural networks method for large vocabulary continuous speech recognition

Recently a pre-trained context-dependent hybrid deep neural network (DNN) and HMM method has achieved significant performance gain in many large-scale automatic speech recognition (ASR) tasks. However, the error back-propagation (BP) algorithm for training neural networks is sequential in nature and is hard to parallelize into multiple computing threads. Therefore, training a deep neural network is extremely time-consuming even with a modern GPU board. In this paper we have proposed a new acoustic modelling framework to use multiple DNNs instead of a single DNN to compute the posterior probabilities of tied HMM states. In our method, all tied states of context-dependent HMMs are first grouped into several disjoined clusters based on the training data associated with these HMM states. Then, several hierarchically structured DNNs are trained separately for these disjoined clusters of data using multiple GPUs. In decoding, the final posterior probability of each tied HMM state can be calculated based on output posteriors from multiple DNNs. We have evaluated the proposed method on a 64-hour Mandarin transcription task and 309-hour Switchboard Hub5 task. Experimental results have shown that the new method using clusterbased multiple DNNs can achieve over 5 times reduction in total training time with only negligible performance degradation (about 1-2% in average) when using 3 or 4 GPUs respectively.